1. Field of the Invention
The present invention relates to a projection optical system and an exposure system provided with the projection optical system, and more particularly, to a projection optical system suitable for an exposure system used when microdevices including semiconductor devices are manufactured using a photolithography process.
2. Related Background of the Invention
In recent years, miniaturization has been further advancing in semiconductor device manufacture and semiconductor chip-mounted substrate manufacture. In exposure systems for printing patterns, projection optical systems having higher resolutions have come to be demanded. In order to satisfy such demand for higher resolutions, the wavelength of exposure light (radiation) must be shortened and, at the same time, NA (numerical aperture of a projection optical system) must be enlarged. However, if the wavelength of exposure light is shortened, the types of optical glass capable of being practically used are limited because of light absorption.
For example, when vacuum ultraviolet light having a wavelength of 200 nm or less, particularly F2 laser light (wavelength: 157 nm) is used as exposure light, there is no other choice but to use fluoride crystals including calcium fluoride (fluorite, CaF2) and barium fluoride (BaF2), as light-transmissive optical material constituting a projection optical system in many cases. Practically, the projection optical system is basically presumed to be formed of only fluorite in a design of an exposure apparatus using F2 laser light as the exposure light. It was considered that fluorite was a crystal belonging to the cubic system and optically isotropic, and practically had no birefringence. Moreover, in conventional experiments using visible light, only small birefringence (random one due to internal stresses) was observed for fluorite.
However, in a symposium on lithography held on May 15, 2001 (2nd International Symposium on 157 nm Lithography), John H. Burnett et al. of US NIST presented that they experimentally and theoretically confirmed the existence of intrinsic birefringence in fluorite.
According to this presentation, birefringence in fluorite is approximately zero in the direction of the crystal axis [111] and the directions of its equivalent crystal axes [−111], [1-11], [11-1], and in the direction of the crystal axis [100] and the directions of its equivalent crystal axes [010] and [001], but has practically nonzero values in the other directions. In particular, in the six directions of the crystal axes [110], [−110], [101], [−101], [011], and [01-1], fluorite has a birefringence of up to 6.5 nm/cm for a wavelength of 157 nm and a birefringence of up to 3.6 nm/cm for a wavelength of 193 nm.
These birefringence values are essentially larger than the 1 nm/cm, which is regarded as a permissible value for random birefringence. Moreover, there is a possibility that the influence of birefringence could be accumulated through a plurality of lenses because of the nonrandom distribution. In the conventional art, since the birefringence of fluorite is not taken into consideration in the design of projection optical systems, and the crystal axis [111] and the optical axis are coincided from the standpoint of ease in machining in general. In this case, the NA (numerical aperture) is relatively large in a projection optical system, and consequently, light beams inclined in a certain degree from the crystal axis [111] also pass through the lenses. As a result, the image-forming performance may be deteriorated due to the influence of birefringence.
An object of the present invention is to provide a projection optical system having a favorable optical performance without being practically influenced by birefringence even if optical material having birefringence (particularly, intrinsic birefringence), such as fluorite, is used. Moreover, another object of the present invention is to provide an exposure system and an exposure method capable of performing projection and exposure with high resolution and high precision using the projection optical system of the present invention which has a favorable optical performance without being practically influenced by birefringence.